In injection-molding machines using oil pressure for drive sources for its injection unit, clamping (mold clamping) unit, etc., the strokes of a screw for injection and a clamp axis depend on the stroke of associated hydraulic cylinders, and the origin positions or zero points of the screw and the clamp axis are determined mechanically by themselves.
In injection-molding machines driven by means of servomotors, however, the rotation of the servomotors is subject to no mechanical restrictions. Therefore, the origin positions of an injection axis, clamp axis, and ejector axis, which are driven by these servomotors, cannot be uniformly determined mechanically. Unless the origin points of the axes driven by the servomotors are positioned accurately with respect to the body of the injection-molding machine, however, the injection-molding machine body and equipment associated therewith may be possibly damaged. Unless the position of the injection axis, for example, or the screw position, relative to the injection-molding machine body, is detected accurately, and if the screw is not positioned accurately, the screw may possibly run against a heating cylinder, thereby damaging the screw or heating cylinder. Also in cushion amount adjustment or in injection-speed switching control, the position of the screw relative to the injection-molding machine body must be detected accurately for screw positioning.
Likewise, unless the clamp axis is positioned accurately with respect to the injection-molding machine body, a mold may be possibly damaged.
Generally, zero return operation along an axis driven by means of a servomotor is effected by locating the origin of the axis at a reference point which is preset on a predetermined coordinate position in a coordinate system of the machine body. The reference point is set to the position which is reached by the axis when the servomotor is rotated through a predetermined angle from a rotational position corresponding to an invariable coordinate position in the coordinate system of the injection-molding machine. More specifically, the injection-molding machine typically comprises a position detecting system which includes an absolute-value pulse coder adapted to deliver a signal indicative of one revolution of the servomotor with every arrival of a grid at a predetermined rotational position, a deceleration dog attached to the injection-molding machine body, and a sensor attached to the axis for detecting the decelerated dog. The position detecting system is adjusted so that the grid of the pulse coder takes a position opposite to the predetermined rotational position when a deceleration dog signal, delivered from the sensor, goes low while the axis is passing the deceleration dog during the return to the origin. Meanwhile, the reference point is set to the position reached by the axis when the servomotor makes a half turn after the deceleration dog signal diminishes.
Accordingly, if the axis position after the zero return is deviated from the reference point during fine adjustment or the like of gears, driving belt, etc. of a drive system for the axis, the rotational position of the servomotor, after the zero return, is at a halfturn distance, in both positive and negative directions, from the position corresponding to the reference point, thus falling within a one-revolution range. Zero point adjustment can be performed within this range.
In this zero adjustment, the axis is moved toward the reference point, whereupon the deceleration dog signal diminishes. When a one-revolution signal is then produced, the axis is stopped. If the stop position of the axis is deviated from the set reference point, the position of the deceleration dog or the sensor for deceleration dog detection, e.g., a limit switch, is manually corrected by shifting the location of the dog or the sensor so that the stop position coincides with the set reference point.
In machine tools, robots, etc., whose axes are driven by means of servomotors, their accuracy is subject to no special problems, in general, even though the zero return is based on the conventional method as aforesaid. In injection-molding machines, however, the positional relationships between the machine body and the axes to be driven require a strict accuracy in microns. Therefore, the individual axes must be positioned accurately with respect to the coordinate system of the injection-molding machine body. Thus, accurate zero return is needed.
According to the aforementioned system in which the axes are positioned manually at the reference position, however, high-accuracy zero adjustment cannot be achieved. Moreover, the adjustment requires much time, and cannot be performed automatically.